.I have to explain.
This statement is a logical contradiction - but only paradoxically, until it is expanded. In my personal experience, beliefs and thoughts are harmful causing divisions, hatred and in the most extreme cases make people fight each other - if and only if we identify with the "thing" inside our brains that actually does the believing and thinking! Likewise, the statement "Beliefs are harmful" is also harmful if we identify with it! Once the mind realizes that "The thoughts in my head are not Me" then the paradox disappears.
I consider heretical mindset to be a good thing and one of the main factors, even a precondition behind creativity in general; in science, technology and art in particular.